In a table driven, virtual storage network, a table maps virtual disk segments to physical storage locations. A key issue in forming the virtual storage network is the selection of a method to map the virtual storage to the actual storage. For example, a virtual storage network can map fixed-sized segments of contiguous blocks in each mapping table entry. For mapping efficiency, these segments need to be fairly large, e.g., on the order of one megabyte of virtual disk data. One problem with using large segments is the relatively long time period needed to copy a large block of the underlying non-virtual storage during the copying or migration of data. During such data movement operations, the data segments being moved cannot be accessed, and any virtual disk input/output (I/O) operations to these segments must be delayed until data movement is finished. Such delay is typically accomplished by setting a state in the mapping table entry to prevent I/O operations to the effected segment during data movement. For large segments, however, the delays can add unacceptably long latencies to the virtual disk I/O operations and adversely affect performance of the storage system.
The problem of undesirably large latencies during data movement operations may be solved by using smaller segments. Unfortunately, the use of smaller data segments increases the costs associated with mapping, such as the overhead of storing and managing a much larger number of map entries. A small data segment configuration also potentially reduces the amount of contiguous data on the non-virtual storage, causing fragmented storage.
An ideal virtual storage system, therefore, has a mapping system that achieves the benefits for both large segment and small segment mapping. In particular, an ideal mapping system would achieve the low administrative cost of larger storage segmentation and the reduced latencies of smaller storage segmentation.